Two gonadotropic hormones (GTH), GTH I and GTH II, have been isolated and characterized from salmonid pi-tuitaries. GTH II is homologous to LH, and GTH I is often regarded as homologous to FSH, although this is still controversial. Despite the uncertainty, the terms LH and FSH are used in the present paper. During sexual development, the plasma levels of FSH peak concomitantly with elevated plasma androgen levels and spermatogenesis in male and with elevated plasma estradiol levels and vi-tellogenesis in female salmonids. LH, on the other hand, reaches plasma and pituitary peak levels later, when levels of the progestin 17a,20(B-dihydroxy-4-pregnen-3-one (17,20P) are high in both sexes and when males spermiate and females ovulate. A similar pattern is followed by the pituitary FSH content in masu salmon, Oncorhyn-chus masou. In males, the two GTHs can stimulate androgen production and gonadal growth in the earlier stages of maturation with similar effectiveness, whereas LH is more effective than FSH in stimulating production of 17,20P and androgens in the later stages.